Andrew Tarantola

Unlike renegade mammals that live fast and play by their own thermal rules, cold-blooded reptiles and insects march to the beat of a more universal drum—ambient air temperature. You can see it in the speed at which ants walk and fireflies flash, or hear it in the cadence of a cricket's love song. Their melodies are so precise, in fact, that you can even measure the temperature outside by how fast they chirp.

Let's talk a bit about how crickets make their symphony of chirps in the first place. They employ a method known as stridulation, which is surprisingly common throughout the Earth's cold-blooded populations, from insects like beetles and crickets to arachnids such as tarantulas, reptiles including the Desert Viper, and even birds like the Club-winged Manakin. Stridulation is, in essence, the sound an animal makes by rubbing two body parts together.

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For the most part, male crickets alone produce sounds, and do so by grating the ridged underside of one wing against a special structure, known as a file, located atop the edge of the opposite wing. Think of it like playing a tiny, biological washboard.

How fast they strum those little washboard bodies of theirs depends entirely on the weather. Being cold-blooded animals, in fact, all of their actions are directly correlated to conditions in the surrounding environment—the warmer it is outside, the faster crickets move, and the more often they'll sing. Think of it like you would a common chemical reaction: the more thermal energy you pump into a system, the faster and more vigorously molecules will move and collide within it, whether it's milk curdling on a hot car dashboard versus a refrigerator shelf or a cricket gathering enough activation energy within its muscles to instigate a chirp.

How much the temperature affects a cricket's rate of chirping can be calculated using the Arrhenius equation:

k = Ae-Ea/(RT)

In this equation the pre-exponential factor ("A") represents the fraction of molecules that would react if either the activation energy ("Ea") were zero. That is, it measures the temperature-modulated frequency that molecules within a system will collide at the right speed and angle to instigate a reaction. The exponential portion ("e-Ea/(RT)") illustrates the proportion of reactant molecules in the system that still possess enough energy to pop versus the number of already-exhausted molecules, wherein Ea is the activation energy needed to push the reaction into its transition state, R is the gas constant, and T is the temperature of the system measured in Kelvin. Here it is in video form, if you prefer to watch your math fly before your eyes instead of stare at it on a page.

As the UC Davis Chemwiki explains:

The exponential term in the Arrhenius Equation implies that the rate constant of a reaction increases exponentially when the activation energy decreases. Because the rate of a reaction is directly proportional to the rate constant of a reaction, the rate increases exponentially as well. Because a reaction with a small activation energy does not require much energy to reach the transition state, it should proceed faster than a reaction with a larger activation energy.

Again, this is why milk curdles faster and crickets sound off more often in hot weather than in cold. The equation was originally developed in the Victorian Era by Swedish scientist Svante Arrhenius as a means of explaining the temperature-dependence of equilibrium constants that his contemporaries were observing in their experiments.

How to Use Crickets as a Natural Thermometer

So now that your eyes have thoroughly glazed over with all the talk of activation energies and transition states, here's how you can leverage the Arrhenius equation and a cricket into telling you the exact temperature outside. Since the ambient temperature dictates the speed at which a cricket chirps, you can figure out how hot it is by counting the cricket's chirp speed.

First thing you need is a captive cricket—either one from your yard or from the pet store where they're sold as live food for spiders and reptiles—as well as a stopwatch. Count the cricket's chirps for 14 seconds and note the total. Repeat this twice more for a total of three, 14-second intervals, then average the three totals. Take that number and add 40, the resulting number should within one degree Fahrenheit of the ambient air.

If you live in a country where the temperature is measured in Celsius, the method is slightly different. Use a 25 second interval instead. Take that total, divide it by three and add four:

T = (Chirps/3) + 4

And voila, you have your local temperature. It may a bit more involved than just pulling up an app of staring at some mercury, but it's also fun way to bring science to life. So grab your kids (or borrow your neighbor's), head on out to the back yard learn 'em a thing or two. [Wiki - Scientific American - UC Davis - Dartmouth]